I was struggling to wrap my head around how federated social media works until I realized that email has basically been doing the same thing for 30 years. Different email servers are like instances of a federated network. You can send emails to people from within a single server or you can send emails to people on any other mail server. Your email address is a username followed by an ‘@’ and the server address, just like on Lemmy. Email is a decentralized service I’ve been using the whole time!
Does each lemmy server need to constantly pull and store all activity from all other instances? That’d be the huge difference here, but I’m not sure how the linking works exactly. Imagine your tiny email server having to sync with all activity on gmail…
The details don’t match but the overall architecture is similar. Obviously since email is just messaging between two parties you don’t have to download anything, the server just waits until a message comes in.